Endoscopic instruments are presently used in many fields of medicine, but they are also used in engineering, where they are known as so-called technoscopes. In recent years, a large number of applications have been made possible only as a result of the fact that significant progress has been made in endoscopic technology. Thus, there are already endoscopes known today that have diameters that amount to only a few millimeters in order to be able to penetrate even the smallest clearances.
Rod lenses are often used in long, stretched out, thin endoscopes of this type. Such rod lenses can have a diameter of 2.78 mm, for example, with a total length of 28 mm. They sit inside a tubular shaft, wherein the shaft is, of course, kept as thin as possible and only as thick as necessary in terms of its wall thickness in order to keep the overall cross-sectional dimensions as small as possible.
With such endoscopes, it is hardly possible to avoid the fact that, in spite of the protected position of the rod lenses within the shaft, when the shaft bends, forces act upon these rod lenses as well. In practice, it is thus not possible to avoid the fact that the optic that lies inside the shaft, a rod lens in particular, is placed under stress by external bending loads and can possibly be damaged in this way. If such a lens breaks, it destroys the complete endoscope. Because of this, in practice, there are limits to the development of endoscope optics with external dimensions that are as small as possible.